H OH Haworth
H OH Haworth
1C4 Less Stable
CH2OH Fischer hoh2c hoh2c
HO H Haworth
Figure 1-4. Representations of the cyclic structures of ß-d-giucopyranose and ß-d-fructofuranose. The chair conformations are designated "C" with the superscript number on the left, indicating the number of the sugar carbon that lies above the plane, and with the subscript number on the right, indicating the number of the sugar carbon that lies below the plane. The other three ring carbons and the ring oxygen lie within the plane.
carbon chain in the Fischer projection are below the plane of the ring in the Haworth formula. The exocyclic group (the —CH2OH group of aldohexopyranoses, for example) is placed above or below the plane in the Haworth formula, depending on the origin of the ring oxygen (C-4 or C-5 for glucofuranose or pyranose, respectively, and C-5 for fructofur-anose). If the ring connects from an oxygen on the right of the carbon chain in the Fischer structure, then the exocyclic group is above the plane in the Haworth structure.
Because five- and six-membered rings are not planar, the conformational formula is preferred for showing spatial relationships, such as in enzyme-catalyzed reactions where fit of substrate to enzyme binding site is important. In chair conformations, carbons 2, 3, 5, and the ring oxygen are planar, and carbons 1 and 4 are out of the plane and on opposite sides of the plane. The orientations of the hydroxyls are axial or equatorial. Pyranose sugars assume a chair conformation based in part on maximizing the number of large groups (—OH and —CH2OH) at equatorial positions, which are less sterically hindered than are axial positions. Thus, the 4Q conformation, in which C-4 is above and C-l is below the plane, is preferred (lower energy) for a- and (3-d-glucopyranose and most of the other aldo-hexoses.
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